The present invention relates to a fuel pump device with temperature regulation.
One way of reducing discharges of emissions from diesel engines is to inject the fuel at a very high pressure. A so-called “Common Rail” system is commonly used for effecting injection at a high pressure in the combustion spaces of a diesel engine. A Common Rail system comprises a high-pressure pump which pumps fuel at a high pressure to an accumulator tank (“Common Rail”). The fuel in the accumulator tank is intended to be distributed to all the cylinders of the combustion engine. Fuel from the accumulator tank is injected into the combustion spaces of the respective cylinders by electronically controlled injection means.
When a high-pressure pump pressurises the fuel, a certain fuel leakage inevitably occurs at the clearance between the pressure-generating components of the fuel pump, which usually take the form of a piston and a cylinder. The amount of the fuel leakage is related to the efficiency of the fuel pump. A high-pressure pump is normally provided with a piston made at least partly of very wear-resistant material, e.g. ceramic material. Such wear-resistant material usually has a lower thermal expansion coefficient than the metal material normally used in the cylinder. When the high-pressure pump is operating, the energy supplied is partly used for pressurising the fuel, while the remainder converts to heat energy, some of which warms the piston and the cylinder. The fact that the material of the piston has a lower thermal expansion coefficient than the material of the cylinder results in the clearance between the piston and the cylinder increasing as they become warmer. The increasing clearance leads also to the fuel leakage between piston and cylinder increasing with temperature. The fuel leakage itself also gives rise to further heating of the piston and the cylinder when fuel flows at high velocity through the clearance in contact with the surfaces of the piston and the cylinder. This further heating of the piston and the cylinder increases the clearance further, resulting in still greater fuel leakage. During operation of conventional high-pressure pumps there is therefore relatively great heating of the piston and the cylinder, resulting in a large fuel leakage flow and reduced efficiency of the fuel pump. The high fuel pressure may itself also cause expansion of the cylinder with consequently increased clearance between the surfaces of the piston and the cylinder. This expansion of the cylinder is also temperature-dependent.